The present invention relates to automatic visibility improvement method for digital image suited for electronic imaging devices such as video cameras and digital cameras.
When an image of a scene having very dark and very bright parts (see FIG. 8(a) or FIG. 12(a)) is picked up with an electronic imaging device such as a video camera or a digital camera and is displayed on a monitor or outputted to a printer, the very bright parts tend to be decolored while the very dark parts tend to be merged with back ground (see FIG. 8(b) or FIG. 12(b)). Consequently, there arises a phenomenon of failure of reproduction of detail data obtained with imaging element of electronic imaging device. This phenomenon stems from the fact that the image output apparatus such as monitor or printer has a narrower dynamic range than that of the image inputted to the image output apparatus.
Various methods have heretofore been proposed to reduce the above data loss. For example, Japanese Patent No. 2951909 discloses a system for and a method of gradation correction for imaging devices, in which an input image is divided into a plurality of blocks of a square grating, the average brightness of the blocks is then computed, and gradation correction is made for each division area on the basis of the computed average brightness.
However, according to the above Japanese Patent No. 2951909, since the input image is divided into a plurality of division areas on the basis of the average brightness of the blocks, a problem arises, for example, where a texture A as shown in FIG. 9(a), in which the brightness varies stepwise in a narrow brightness range, and a texture B as shown in FIG. 9(b), in which the brightness varies stepwise in a broad brightness range, are adjacent areas. In this case, the two areas are regarded to be the same and are gradation corrected with the same gradation correction curve. Consequently, if the gradation correction curve is determined to be optimal for the texture A, the texture B is decolored in very bright parts and indistinguishable in very dark parts. If the curve is determined to be optimal for the texture B, on the other hand, the stepwise brightness variations of the texture A may not be reproduced.
In addition, although no detailed description of the gradation correction was made, if the method thereof is like those in prior arts, the gradation correction will be executed such that density values that frequently appear in the area tend to be highly emphasized in terms of contrast. This means that useful data maybe lost when their density values do not appear frequency.